Aqueous polytetrafluoroethylene dispersion composition

ABSTRACT

A composition comprising from 30 to 65 wt % of fine particles of PTFE, and, based on PTFE, from 2 to 12 wt % of C10H21CH(CH3)CH2O(C2H4O)9C3H6OH and at least 0.01 wt % and less than 1 wt % of a polyethylene oxide having an average molecular weight of from 100,000 to 2,000,000.

TECHNICAL FIELD

The present invention relates to a PTFE aqueous dispersion compositioncontaining fine particles of polytetrafluoroethylene (hereinafterreferred to as PTFE).

BACKGROUND ART

PTFE by an emulsion polymerization method is usually produced byinjecting and polymerizing tetrafluoroethylene (hereinafter referred toas TFE) monomer under pressure while adding and stirring a peroxide typepolymerization initiator, an anionic dispersant and a higher paraffin orthe like as a polymerization stabilizer, to pure water. Usually, anaqueous dispersion will be obtained wherein fine particles of PTFEhaving an average particle size of from 0.1 to 0.5 μm are dispersed at aconcentration of about 30 wt % or lower.

This aqueous dispersion is very unstable by itself, and it has beencommon that a nonionic surfactant such as a polyoxyethylene alkyl phenylether, e.g. Triton X 100, manufactured by Union Carbide having amolecular structure of C₈H₁₇C₆H₄O(C₂H₄O)₁₀H, is added for primarystabilization, followed by concentration to a PTFE solid content of from40 to 65 wt % by a known method such as an electrical concentrationmethod or a phase separation method. Thereafter, in order to obtain aliquid physical property such as a viscosity suitable for variousapplications or stabilization during storage for a long period of time,water, an antiseptic such as ammonia, a surfactant, etc., are added toobtain an aqueous dispersion composition having a PTFE concentration offrom 30 to 65 wt %.

However, there has been a problem that if a frictional action or ashearing action is exerted to the conventional PTFE aqueous dispersioncomposition, PTFE particles tend to be fibrous and agglomerate, wherebythe uniformity of the dispersion tends to be impaired.

Specifically, when a PTFE aqueous dispersion composition is transportedor circulated by a pump, if the operation time prolongs, agglomerates ofPTFE are likely to be formed in the PTFE aqueous dispersion compositionby a frictional action or a shearing action, whereby there has been aproblem that the pump is likely to be clogged, the valve operation islikely to be impaired, or the filter is likely to be clogged.

Further, there has been a problem that when an excess PTFE aqueousdispersion composition coated on a substrate such as a glass fiber clothor a metal foil, is to be removed by a doctor bar, agglomerates of PTFEformed by a frictional action, tend to deposit on the substrate, thusleading to drawbacks such as irregularity in thickness or foreignmatters to deteriorate the yield of the product.

JP-B-53-6993 proposes a method of incorporating at least 1 wt % of analkylene oxide polymer to PTFE for the purpose of preventingsedimentation of fine particles of PTFE, but discloses nothing aboutstability against the frictional action or the shearing action of theliquid. The polyethylene glycol having a relatively low molecular weighti.e. a molecular weight of from 1300 to 15000 disclosed in Examples ofthis specification was not one having an adequate effect for improvingthe frictional stability.

Further, a film prepared from a PTFE aqueous dispersion compositionemploying a conventional polyoxyethylene alkyl phenyl ether typesurfactant, has had a problem that it is likely to be colored, and theelectrical characteristic such as an insulating property tends todeteriorate.

JP-A-8-269285 discloses that a film prepared from an aqueous dispersioncomposition employing a polyoxyethylene alkyl ether type surfactanthaving a cloud point of from 48 to 85° C. and a content of ethyleneoxide groups of from 65 to 70 wt %, such as RO(C₂H₄O)_(n)H (wherein R isa C₈₋₁₈ linear alkyl group, and n is from 5 to 20), has colorationsuppressed. However, this aqueous dispersion composition has thefollowing problem. Namely, foams formed by stirring are hardlyextinguishable, and if such foams will attach, traces of foams willbring about irregularities in the thickness of the PTFE film, and thestrength, etc. of the film may deteriorate, thus leading tonon-uniformity, and when the aqueous dispersion composition is subjectedto multi-coating to prepare a thick film, the sintered PTFE surface hasa low surface tension and is repelling, whereby multi-coating tends tobe difficult.

DISCLOSURE OF THE INVENTION

The present invention has an object to improve a PTFE aqueous dispersioncomposition excellent in friction stability or shearing stability.

The present invention has an object to provide a PTFE aqueous dispersioncomposition whereby foams formed by stirring are readily extinguishableand multi-coating is possible, and a sintered PTFE film is free fromcoloration.

The present invention is a polytetrafluoroethylene aqueous dispersioncomposition, which comprises from 30 to 65 wt % of fine particles ofpolytetrafluoroethylene obtainable by emulsion polymerization, from 2 to12 wt %, based on the polytetrafluoroethylene, of a nonionic surfactant,and at least 0.01 wt % and less than 1 wt %, based on thepolytetrafluoroethylene, of at least one member selected from apolyethylene oxide having an average molecular weight of from 100,000 to2,000,000 and an association type thickener based on a water-solublepolyurethane as essential components.

The present invention is the polytetrafluoroethylene aqueous dispersioncomposition, wherein the nonionic surfactant is at least one memberselected from nonionic surfactants having molecular structuresrepresented by the Formulae (1) and (2):

R—O—A—H  (1)

(wherein R is an alkyl group having a straight chain or a branched chainrepresented by C_(x)H_(2x+1) where x=8 to 18, and A is a polyoxyalkylenechain constituted by from 5 to 20 oxyethylene groups and from 0 to 2oxypropylene groups)

R′—C₆H₄—O—A′—H  (2)

(wherein R′ is an alkyl group having a straight chain or a branchedchain represented by C_(x′)H_(2x′+1) where x′=4 to 12, and A′ is apolyoxyethylene chain constituted by from 5 to 20 oxyethylene groups).

Further, the present invention is the PTFE aqueous dispersioncomposition, wherein the nonionic surfactant is a nonionic surfactanthaving a molecular structure represented by the Formula (1) wherein x isfrom 10 to 16, and A is a polyoxyalkylene chain constituted by from 7 to12 oxyethylene groups and from 0 to 2 oxypropylene groups.

Further, the present invention is the PTFE aqueous dispersioncomposition, wherein the nonionic surfactant is a nonionic surfactanthaving a molecular structure represented by the Formula (2) wherein x′is from 10 to 16, and A′ is a polyoxyalkylene chain constituted by from7 to 12 oxyethylene groups.

The present invention is a polytetrafluoroethylene aqueous dispersioncomposition, which comprises from 30 to 65 wt % of fine particles ofpolytetrafluoroethylene obtainable by emulsion polymerization, and from2 to 12 wt %, based on the polytetrafluoroethylene, of a nonionicsurfactant having a molecular structure represented by the Formula (3):

R—O—B—H  (3)

(wherein R is an alkyl group having a straight chain or a branched chainrepresented by C_(x)H_(2x+1) where x=8 to 18, and B is a polyoxyalkylenechain constituted by from 5 to 20 oxyethylene groups and from 1 to 2oxypropylene groups).

Further, the present invention is the PTFE aqueous dispersioncomposition, wherein the nonionic surfactant is a nonionic surfactanthaving a molecular structure represented by the Formula (3) wherein x isfrom 10 to 16, and B is a polyoxyalkylene chain constituted by from 7 to12 oxyethylene groups and from 1 to 2 oxypropylene groups.

BEST MODE FOR CARRYING OUT THE INVENTION

The fine particles of PTFE in the PTFE aqueous dispersion composition ofthe present invention are those obtainable by emulsion polymerization,and they can be produced by a conventional emulsion polymerizationmethod.

The average particle size of the fine particles of PTFE is usuallywithin a range of from 0.1 to 0.5 μm, particularly preferably within arange of from 0.15 to 0.3 μm.

Further, the average molecular weight of PTFE is suitably within a rangeof from a 1,000,000 to 100,000,000. If it is too small, the strength ofPTFE tends to be low, and if it is too large, industrial polymerizationtends to be difficult.

Further, the average molecular weight is obtained from a method by Suwaet al. (Journal of Applied Polymer Science, 17, 3253—(1973)) using heatof crystallization obtained by differential thermal analysis.

In the present invention, PTFE includes not only a homopolymer of TFEbut also a so-called modified PTFE which contains a small amount ofpolymer units based on a component co-polymerizable with TFE, such as ahalogenated ethylene such as chlorotrifluoroethylene, a halogenatedpropylene such as hexafluoropropylene or a fluorovinyl ether such as aperfluoro(alkylvinyl ether) and which is not substantiallymelt-processable.

In the PTFE aqueous dispersion composition of the present invention, thecontent of the fine particles of PTFE is from 30 to 65 wt %, preferablyfrom 50 to 62 wt %. If it is lower than the above range, the fineparticles of PTFE tend to be sedimented because of a low viscosity, andif it is higher than the above range, processing tends to be difficultbecause of high viscosity.

In the PTFE aqueous dispersion composition of the present invention, atleast one member selected from a polyoxyalkylene alkyl ether typenonionic surfactant of the above Formula (1) and a polyoxyethylene alkylphenyl ether type nonionic surfactant of the Formula (2), is employed.In the Formulae (1) and (3), R which is a hydrophobic group, is an alkylgroup, and the value of x representing the chain length of the alkylgroup is within a range of from 8 to 18, preferably from 10 to 16. If xis too small, the PTFE aqueous dispersion composition tends to have ahigh surface tension, whereby the wettability tends to be low.Inversely, if x is too large, when the dispersion is left to stand for along time, the fine particles of PTFE are likely to be sedimented,whereby the storage stability of the PTFE aqueous dispersion compositiontends to be impaired.

Further, in the Formulae (1) and (3), if the alkyl group as ahydrophobic group, has a branched structure, the wettability will beimproved, and a suitable aqueous dispersion composition can be obtained,such being preferred.

In the Formula (1), A which is a hydrophilic group, is a polyoxyalkylenechain constituted by from 5 to 20 oxyethylene groups and from 0 to 2oxypropylene groups. A polyoxyalkylene chain constituted by from 7 to 12oxyethylene groups and from 0 to 2 oxypropylene groups is preferred fromthe viewpoint of the viscosity and the stability. Particularly preferredis a case where it has from 0.5 to 1.5 oxypropylene groups in thehydrophilic group A, whereby the defoaming property will be good.

In A, the oxypropylene groups may be present in the polyoxyethylenechain or may be bonded to the polyoxyethylene chain terminals. However,when they are bonded to the polyoxyethylene chain terminals, thedefoaming property will be good, and particularly when they are bondedto the molecular terminal side of the two polyoxyethylene chainterminals, the defoaming property will be better, such being preferred.

Specific examples of the nonionic surfactant of the Formula (1) include,for example, surfactants having molecular structuresC₁₃H₂₇O(C₂H₄O)₉₋₁₀H, C₁₀H₂₁CH(CH₃)CH₂O(C₂H₄O)₁₀H,C₁₀H₂₁CH(CH₃)CH₂O(C₂H₄O)₉₋₁₀OC₃H₆OH, C₁₂H₂₅O(C₂H₄O)₁₀H,C₁₆H₃₃O(C₂H₄O)₁₀₋₁₁H, and C₁₀H₃₃O(C₂H₄O)₆₋₇H (provided that an alkylgroup other than the branched structural moiety is of a straight chainstructure, and the same applies in the following specific examples). Thenonionic surfactants, of the Formula (1) may be used alone or as aplural mixture of them.

In the Formula (2), R′ as a hydrophobic group, is an alkyl group, andthe value of x′ representing the chain length of the alkyl group is from4 to 12, preferably from 6 to 10. If x′ is too small, the PTFE aqueousdispersion composition tends to have a high surface tension, whereby thewettability tends to be low. Inversely, if x′ is too large, when thedispersion is left to stand for a long time, the fine particles of PTFEare likely to be sedimented, whereby the storage stability tends to beimpaired.

Further, A′ which is a hydrophilic group, is a polyoxyethylene chainhaving from 5 to 20 oxyethylene groups. From 7 to 12 oxyethylene groupsare particularly preferred from the viewpoint of the viscosity and thestability.

Specific examples of the nonionic surfactant of the Formula (2) include,for example, C₉H₁₉C₆H₄O(C₂H₄O)₉₋₁₀H and C₈H₁₇C₆H₄O(C₂H₄O)₉₋₁₀H. Thenonionic surfactants of the Formula (2) may be used alone or as a pluralmixture of them.

Among the nonionic surfactants of the Formulae (1) and (2) in thepresent invention, it is novel to use a compound of the Formula (3) as anonionic surfactant in a PTFE aqueous dispersion composition. By the useof the nonionic surfactant of the Formula (3), it is possible to obtaina PTFE aqueous dispersion composition whereby foams formed by stirringare readily extinguishable, multi-coating is possible, and a sinteredPTFE film is free from coloration.

In the Formula (3), B which is a hydrophilic group, is a polyoxyalkylenechain constituted by from 5 to 20 oxyethylene groups and from 1 to 2oxypropylene groups, particularly preferably a polyoxyalkylene chainhaving from 7 to 12 oxyethylene groups and from 1 to 2 oxypropylenegroups from the viewpoint of the viscosity and the stability.

In B, the oxypropylene groups may be present in the polyoxyethylenechain or may be bonded to the terminals of the polyoxyethylene chain.However, when they are bonded to the terminals of the polyoxyethylenechain, the defoaming property will be good, and particularly when theyare bonded to the molecular terminal side of the two polyoxyethylenechain terminals, the defoaming property will be better, such beingpreferred.

When B contains no oxypropylene group, the defoaming property tends tobe low. When oxypropylene groups are three or more, the surface tensiontends to be high, and the wettability tends to be low, and at the timeof multi-coating, “cissing” or “pockmarks” is likely to result.

Specific examples of the nonionic surfactant of the Formula (3) include,for example, surfactants having molecular structures such asC₁₃H₂₇O(C₂H₄O)₈C₃H₆OH, C₁₃H₂₇O(C₂H₄O)₉C₃H₆OH, C₁₃H₂₇O(C₂H₄O)₁₀(C₃H₆O)₂H,and C₁₆H₂₇(C₂H₄O)₁₂(C₃H₆O)₂H. The nonionic surfactants of the Formula(3) may be used alone or as a plural mixture of them.

Further, the molecular structure of the nonionic surfactant is a mixtureof plural substances having a certain distribution, and the carbonnumber of the alkyl group in the nonionic surfactants and the numbers ofthe oxyethylene groups and the oxypropylene groups in the oxyalkylenegroup will be represented by average values. Each numerical value is notlimited to an integer.

In the present invention, the nonionic surfactant to be used forimprovement of the friction stability may be the nonionic surfactant ofthe Formula (1) alone, the nonionic surfactant of the Formula (2) aloneor a combination of the nonionic surfactant of the Formula (1) and thenonionic surfactant of the Formula (2).

Further, in the present invention, when the nonionic surfactantrepresented by the Formula (1) is used, the effect for improvement ofthe friction stability is particularly remarkable.

Further, in the PTFE aqueous dispersion composition of the presentinvention, the content of the nonionic surfactant of the Formula (1),(2) or (3) is from 2 to 12 wt % based on the fine particles of PTFE. Ifit is smaller than this range, the fine particles of PTFE tends toagglomerate due to a mechanical stress such as stirring, whereby thestability tends to deteriorate. If the surfactant is incorporated in alarge amount, such will be suitable for an application for thickcoating, but if it is too much, the thermally decomposed gas of thesurfactant will increase at the time of sintering, thus causing a badodor, and microcracks are likely to form in the coated layer.Preferably, it is from 4 to 12 wt %.

In the present invention, a polyethylene oxide is employed as a frictionstabilizer. The average molecular weight of the polyethylene oxide iswithin a range of from 100,000 to 2,000,000, preferably within a rangeof from 100,000 to 1,000,000. This polyethylene oxide can be obtained bypolymerizing ethylene oxide, or by adjusting the molecular weight ofsuch a polymer by applying a radiation, an electron ray, heat, amechanical stress or other means to the polymer to suitably cause themain chain scission. Further, the polyethylene oxide may be onecontaining propylene oxide or other copolymer units in a degree not toimpair the water-solubility.

If the average molecular weight is larger than the above range,so-called stringiness, a string-forming phenomenon of the aqueousdispersion, tends to occur, whereby processing tends to be difficult,and the viscosity reduction or the property change of the aqueousdispersion tends to be substantial, such being undesirable. And, if itis small, it tends to be difficult to obtain an adequate effect forfriction stability.

The added amount of the polyethylene oxide is at least 0.01 wt % andless than 1 wt %, particularly preferably from 0.1 to 0.5 wt %, based onPTFE. If the added amount is too small, the friction stability tends tobe low, and if it is too large, the thermal decomposition product at thetime of sintering tends to be substantial, and the viscosity of theliquid tends to increase, such being undesirable.

Further, if the polyethylene oxide is added to the PTFE aqueousdispersion composition, so-called clusters are likely to form byinclusion of air during the dissolution. Accordingly, a method ofpreliminarily dissolving or dispersing it in water or in a water-solubleorganic solvent such as isopropyl alcohol, propylene glycol, ethyleneglycol or polyethylene glycol, and then adding it, may suitably beselected.

In the present invention, an association type thickener based on awater-soluble polyurethane is employed as a friction stabilizer. Theassociation type thickener based on a water-soluble polyurethane isobtainable by reacting a polyisocyanate with a polyalkylene etheralcohol or by reacting a polyisocyanate with a polyalkylene ether polyoland a polyalkylene ether alcohol. It is one having at least twohydrophobic groups and a hydrophilic group made of a polyoxyalkylenecomposed mainly of oxyethylene groups with a polymerization degree offrom 1 to 500. As its specific examples, JP-A-9-71766 and JP-A-9-71767disclose compounds represented by the following molecular Formulae:

(wherein R¹ is a m-valent hydrocarbon group, each of R² and R⁴ which maybe the same or different from each other, is an alkylene group, R³ is a(h+1)-valent hydrocarbon group which may have a urethane bond, R⁵ is abranched hydrocarbon group, m is a number of at least 2, h is a numberof at least 1, each of k and n is a number of from 1 to 500, and R² andR⁴ are preferably ethylene groups.)

(wherein R⁶ is a q-valent hydrocarbon group which may have a urethanebond, R is an alkylene group, R⁸ is a branched hydrocarbon group, p is anumber of from 1 to 500, q is a number of at least 1, and R⁷ ispreferably an ethylene group.)

Further, as a commercial product, Adecanol UH series (manufactured byAsahi Denka Kogyo K.K.), TAFIGEL PUR series (manufactured by MUNZINGCHEMIE GMBH) or SN thickener 603, 604, 612 or 614 (manufactured bySUN-NOPCO K.K.) may, for example, be used.

The added amount of the association type thickener based on thewater-soluble polyurethane is at least 0.01 wt % and less than 1 wt %,based on PTFE. It is particularly preferably from 0.1 to 0.5 wt %. Ifthe added amount is too small, the effect for friction stabilizationtends to be small, and if it is too large, the thermal decompositionproduct during sintering tends to be substantial, and the viscosity ofthe liquid tends to increase, such being undesirable.

The reason for the improvement of the friction stability of the PTFEaqueous dispersion composition by the addition of the above polyethyleneoxide or the association type thickener based on the water-solublepolyurethane, is not clearly understood. It is conceivable that when africtional stress or a shearing stress is exerted to the fine particlesof PTFE, the particles will slip among themselves by the lubricatingfunction of the stabilizer, whereby coagulation by contact of the PTFEparticles one another will be prevented. It is considered thatpolyethylene glycols or other thickeners having relatively low molecularweights have low effects for improvement of the friction stability,since their lubricating functions are small. Further, among polymers ofethylene oxide, those having molecular weights of at most about 20,000,are usually called polyethylene glycols and are distinguished from thosehaving larger molecular weights, which are called polyethylene oxides.The polyethylene oxide and the association type thickener based on thewater-soluble polyurethane to be used as friction stabilizers, may beused in combination.

To the PTFE aqueous dispersion composition of the present invention,components such as fluorine type or silicone type nonionic surfactants,anionic surfactants, thixotropic agents, various salts, water-solubleorganic solvents, antiseptics such as ammonia, various leveling agents,coloring agents, pigments, dyes or fillers, may be optionally added, asthe case requires.

The viscosity of the PTFE aqueous dispersion composition of the presentinvention is preferably within a range of from 10 to 50 mPs as measuredat 23° C. If it is lower than this range, the PTFE particles are likelyto sediment, whereby the standing stability tends to be impaired, and ifit is higher than this range, the amount, applied at the time ofcoating, tends to be large, such being undesirable.

As a method for producing the PTFE aqueous dispersion composition of thepresent invention, a method may be mentioned wherein the nonionicsurfactant of the Formula (1), (2) or (3) is added to the emulsionpolymerization solution for primary stabilization, followed byconcentration to a PTFE solid content of from 40 to 65 wt % by means ofa known method such as an electrical concentration method or a phaseseparation method, and further by adjustment to a liquid physicalproperties suitable for various applications or for stabilization forstorage for a long period of time, and if necessary, from 0.01 to 1 wt %of an antiseptic such as ammonia, a nonionic surfactant, a frictionstabilizer, etc., are added to obtain an aqueous dispersion compositionhaving a PTFE concentration of from 30 to 65 wt %.

The amount of the nonionic surfactant of the Formula (1), (2) or (3)added to the emulsion polymerization solution for primary stabilization,is preferably from 2 to 12 wt %, particularly preferably from 2 to 8 wt%, based on PTFE.

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples, but these will by nomeans restrict the present invention.

Here, working Examples are Examples 1 to 5 and 12 to 14, and ComparativeExamples are Examples 6 to 11 and 15.

A method for preparing a sample and an evaluation method of each itemwill be shown below.

(A) Surface tension: The surface tension of a PTFE aqueous dispersioncomposition was measured by a ring method using a platinum wire ring.

(B) Viscosity: The viscosity of a PTFE aqueous dispersion was measuredby means of a Brookfield viscometer at a liquid temperature of 23° C.using #1 spindle at 60 rpm.

(C) Friction stability: Using a tube type pump having a TYGON tube withan outer diameter of 7.9 mm and an inner diameter of 4.8 mm mounted, 100cc of a PTFE aqueous dispersion composition in a 200 cc beaker wascirculated for two hours at a liquid transporting amount of 200 cc perminute at room temperature of 23° C., whereupon coagulates werecollected by a 200 mesh nylon filter and the weight of the coagulates,dried at 120° C. for one hour, was measured. Here, if the amount of thecoagulates was 0.6 g or less, the stability was rated as good, and if itwas 1 g or more, the stability was rated as poor.

(D) Thermal stability test: A PTFE aqueous dispersion composition wasleft to stand at 50° C. for 200 hours while simply stirring up and downonce per day, and the viscosity and the friction stability weremeasured.

(E) Stringiness: A PTFE aqueous dispersion composition was dropped byusing a dropping pipette, and the presence or absence of stringing wasvisually determined.

(F) Standing stability: A PTFE aqueous dispersion composition was leftto stand in a 100 ml measuring cylinder at room temperature for twomonths, whereupon the thickness of the formed supernatant layer and thethickness of the residue at the bottom were measured. If such values arenot more than 10 mm upon expiration of two months, the composition canbe used without any practical problem.

(G) Defoaming property: 100 ml of a PTFE aqueous dispersion compositionwas put into a 500 ml measuring cylinder, air was brown thereinto bymeans of a diffuser stone, and after the height of the foam became 200mm, air was stopped, and the composition was left to stand naturally for5 minutes, whereupon the height of the foam was measured.

(H) Thickness of PTFE film: Using an eddy-current thickness tester, theaverage value of measurements at 10 points and the standard deviationwere measured.

(I) Tensile strength: Test specimens were punched out by a microdumbbell, and a tensile test was carried out by means of an Instron typetensile tester at 23° C. with a distance between chucks being 35 mm at atensile speed of 100 mm/min. Five specimens are measured, and theaverage value of the tensile strength and the standard deviation werecalculated. The degree of fluctuations of the tensile strength wasjudged in accordance with the following standards.

Large fluctuations: Standard deviation>10(×10⁶/m² (hereinafterrepresented by MN/m²))

Small fluctuations: Standard deviation<5(MN/m2)

(J) Degree of coloration of e.g. a PTFE film, a glass fiber cloth or thelike:

The object is placed on white paper and visually evaluated.

(K) Sedimentation stability: A PTFE aqueous dispersion composition wasleft to stand in a 100 ml measuring cylinder at room temperature for onemonth, whereupon good or not was judged on such a basis that formationof the supernatant and formation of the sedimented product at the bottomare remarkable or not.

Average molecular structures of surfactants used in the respectiveExamples are shown in Table 1.

TABLE 1 Type of surfactant Average molecular structure Surfactant (a)C₁₀H₂₁CH(CH₃)CH₂O(C₂H₄O)₉C₃H₆OH Surfactant (b) C₁₃H₂₇O(C₂H₄O)₉H(straight chain) Surfactant (c) C₁₂H₂₅O(C₂H₄O)₉H (straight chain)Surfactant (d) C(CH₃)₃CH₂C(CH₃)₂C₆H₄O(C₂H₄O)₉H

EXAMPLE 1

By emulsion polymerization, an aqueous dispersion having an averageparticle size of fine particles of PTFE of 0.25 μm, an average molecularweight of 4,000,000 and a concentration of 20 wt %, was obtained.Surfactant (a) was added thereto in a proportion of 5 wt % based on PTFEfor primary stabilization, the concentration was carried out by anelectrical concentration method, and the supernatant was removed toobtain a concentrated liquid comprising 63 wt % of PTFE and 2.5 wt %,based on PTFE, of surfactant (a).

To this concentrated liquid, 2.5 wt %, based on PTFE, of surfactant (a)was additionally added, water and 200 ppm of ammonia were added, andthen a polyethylene oxide having a molecular weight of 500,000 (reagentmanufactured by Wako Jyunyaku) as a friction stabilizer was added in anamount of 0.4 wt % based on PTFE, followed by stirring for two hours, toobtain a dispersion composition as shown in Table 2.

The pH of this dispersion composition was 9.5, the surface tension waslow at a level of 33.0 (×10⁻³ N/m), and the wettability was good.Further, the viscosity at the initial stage was 29.3 mP·s, and theamount of agglomerates formed in the friction stability test was good ata level of 0.42 g. In the thermal stability test, the viscosity and thefrictional stability were stable without no substantial change from theinitial values. There was no stringing, and the standing stability wasalso good. The composition and the evaluation results of the dispersionare shown in Table 2.

EXAMPLES 2 to 5

Using the same aqueous dispersion obtained by emulsion polymerization asin Example 1, the dispersion compositions as identified in Table 2 wereobtained. The dispersion compositions were evaluated in the same manneras in Example 1. The results are shown in Table 2.

EXAMPLE 6

A dispersion composition was obtained in the same manner as in Example 1except that no friction stabilizer was added. The friction stability ofthis dispersion composition was poor.

EXAMPLES 7 to 11

Dispersion compositions were obtained by adding friction stabilizers asidentified in Table 3. The compositions of the dispersion compositionsand the evaluation results are shown in Table 3.

TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 CompositionPTFE concentration (wt %) 60 57 60 60 60 of the Surfactant Type (a) (a)(b) (c) (d) dispersion Concentration 5 7.5 5 5 6 (wt %/PTEE) FrictionType Polyoxyethylene Polyoxyethylene TAFIGEL Adecanol Polyoxyethylenestabilizer oxide oxide PUR 40 UH140S oxide Average Average Averagemolecular molecular molecular weight: 500,000 weight: weight: 200,0001,000,000 Concentration (wt% of 0.4 0.3 0.2 0.5 0.2 solid content/PTFE)Properties pH 9.5 9.4 9.5 9.4 9.6 of the Surface tension (×10⁻³ N/m)33.0 32.9 33.1 33.3 35.5 dispersion Thermal Initial Viscosity 29.3 28.730.2 30.4 23.1 stability (mPs) test Friction 0.42 0.36 0.44 0.47 0.43stability (amount of agglomerates (g)) After Viscosity 28.7 27.1 29.429.6 22.5 200 hrs (mPs) at 50° C. Friction 0.51 0.54 0.51 0.46 0.50stability (amount of agglomerates (g)) Stringiness (Initial) Nil Nil NilNil Nil Standing Thickness of 7 5 9 8 7 stability supernatant layer (2months (mm) at room Thickness of the 7 5 9 8 7 tempera- sedimented layer(mm) ture) Overall evaluation Good Good Good Good Good

TABLE 3 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11Compo- PTFE concentration (wt %) 60 60 60 60 60 60 sition SurfactantType (a) (a) (a) (a) (a) (d) of the Concentration 5 5 5 5 5 5 dis- (wt%/PTEE) persion Friction Type Nil Polyoxy- Polyoxy- Polyoxy-Hydroxyethyl Nil stabilizer ethylene ethylene ethylene cellulose glycoloxide oxide Average Average Average molecular molecular molecularweight: weight: weight: 6,000 70,000 4,000,000 Concentration (wt % — 2 10.07 0.07 — of solid content/PTFE) Prop- pH 9.5 9.3 9.5 9.5 9.4 9.6erties Surface tension (×10⁻³ N/m) 33.2 33.2 33.1 33.1 33.3 35.5 of theThermal Initial Viscosity 18.8 25.4 28.4 29.7 32.2 19.1 dis- stability(mPs) persion test Friction 3.4 2.1 2.2 0.52 0.62 1.14 stability (amountof agglomerates (g)) After Viscosity 18.7 24.8 25.4 20.2 24.6 19.1 200hrs (mPs) at 50° C. Friction 3.58 2.42 2.29 2.74 0.66 1.13 stability(amount of agglomerates (g)) Stringiness (Initial) Nil Nil NilSubstantial Nil Nil Standing stability Thickness of 8 7 7 7 35 8 (2months at room supernatant temperature) layer (mm) Thickness of 8 7 7 715 8 the sedimented layer (mm) Overall evaluation Poor Poor PoorStringiness Poor Friction friction friction friction observed.sedimentation stability stability stability stability Poor stabilityinferior thermal to Example stability 5

EXAMPLE 12

To the same concentrated liquid obtained by emulsion polymerization asin Example 1, surfactant (a) was added in an amount of 2.5 wt % based onPTFE, and water and 200 ppm of ammonia were added to obtain a dispersioncomposition having a PTFE concentration of 60 wt %, wherein surfactant(a) was 5 wt % based on PTFE. The initial viscosity at 23+ C. of thisdispersion composition was 19.7 mP·s, the pH was 9.4, the surfacetension was 33.4 (×10⁻³ N/m), and the defoaming property was good, asthe height of the foam after 5 minutes in the defoaming test was as lowas 100 mm.

Then, the obtained dispersion composition was coated on an aluminumplate having a length of 20 cm and a width of 15 cm and a thickness of0.2 mm by means of a bar coater, dried at 120° C. for 10 minutes,sintered at 380° C. for 20 minutes and then naturally cooled. On theformed PTFE layer, coating, drying and sintering were further repeatedtwice to form a PTFE layer having a total of three layers multi-coated.This layer was peeled to obtain a PTFE film having a thickness of about33.5 μm. During the coating, no “cissing” or “pockmarks” was observed,and the fluctuation in the thickness of the film was small. The tensilestrength of the film was high, and no yellowing was observed on thefilm. The composition of the dispersion and the evaluation results areshown in Table 4.

EXAMPLES 13 to 15

Using the same aqueous dispersion obtained by emulsion polymerization asin Example 1, the dispersion compositions shown in Table 4 wereevaluated in the same manner as in Example 12. The evaluation resultsare shown in Table 4.

TABLE 4 Example 12 Example 13 Example 14 Example 15 Composition of PTFEconcentration (wt %) 60 57 60 60 the dispersion Surfactant Type (a) (a)(a) (c) Concentration 5 10 3 5 (wt %/PTFE) Properties of Viscosity (23°C.) (mPs) 19.7 22.5 16.2 17.2 the dispersion PH 9.4 9.5 9.3 9.3 Surfacetension (×10⁻³ N/m) 33.4 33.2 33.8 34.4 Height of foam after 5 min (mm)10 10 12 55 Defoaming property Good Good Good Slightly poorMulti-coating 1st layer Good Good Good Good properties 2nd layer GoodGood Good Cissing observed 3rd layer Good Good Good Cissing observedPockmarks Nil Nil Nil Observed Physical Thickness (μm) Average value33.5 37.2 17.2 37 properties of Standard 1.25 1.58 0.62 2.62 the filmdeviation Tensile Average value 33.5 34.1 33.4 31.4 strength (MN/m²)Standard 2.1 2.3 2.2 9.2 deviation Fluctuation in Small Small SmallSubstantial strength Coloring Nil Nil Nil Nil Sedimentation stability(change after 1 month) No problem No problem No problem No problemOverall evaluation Good Good Good Poor

The PTFE aqueous dispersion composition of the present invention has agood friction stability, is substantially free from formation ofagglomerates during the production and use, and thus can be usedsuitably. Further, with the PTFE aqueous dispersion composition of thepresent invention, foams are readily extinguishable, and multi-coatingis possible. During the coating, no “cissing” will be observed, and thesintered PTFE film is free from coloration, and there will be nosubstantial fluctuation in the thickness of the obtained film.

INDUSTRIAL APPLICABILITY

The PTFE aqueous dispersion composition of the present invention isuseful for various conventional applications. For example, it can beused for an application wherein it is impregnated and coated on a glassfiber cloth or a carbon fiber cloth and sintered to obtain a roofmaterial for a film structured building or a high frequency circuitboard, an application wherein it is coated and sintered on a metalplate, then peeled to obtain a film which is used as a capacitordielectric for an electric insulating material, a cell applicationwherein its paste with a powder of an active substance such as manganesedioxide, nickel hydroxide or carbon, is coated on an electrode plate, anapplication wherein it is coated on the surface of a cooking utensil, anapplication wherein a co-precipitate with a filler such as lead isprocessed to a bearing material of no-oil supplying type, an applicationas various binders utilizing fibrillation of PTFE, or an applicationwherein a plastic powder is added thereto to prevent dripping duringincineration of plastics.

What is claimed is:
 1. A polytetrafluoroethylene aqueous dispersioncomposition, which comprises from 30 to 65 wt % of fine particles ofpolytetrafluoroethylene obtained by emulsion polymerization, from 2 to12 wt %, based on the polytetrafluoroethylene, of a nonionic surfactant,and at least 0.01 wt % and less than 1 wt %, based on thepolytetrafluoroethylene, of at least one member selected from the groupconsisting of a polyethylene oxide having an average molecular weight offrom 100,000 to 2,000,000 and a thickener based on a water-solublepolyurethane, wherein the composition has a viscosity of from 10 to 50mPs, and wherein the nonionic surfactant is at least one member selectedfrom the group consisting of nonionic surfactants having molecularstructures represented by the Formulae (1) and (2): R—O—A—H  (1) whereinR is an alkyl group having a straight chain or a branched chainrepresented by C_(x)H_(2x+1) where x=8 to 18, and A is a polyoxyalkylenechain comprising from 5 to 20 oxyethylene groups and from 0 to 2oxypropylene groups R′—C₆H₄—O—A′—H  (2) wherein R′ is an alkyl grouphaving a straight chain or a branched chain represented byC_(x′)H_(2x′+1) where x′=4 to 12, and A′ is a polyoxyethylene chaincomprising from 5 to 20 oxyethylene groups.
 2. Thepolytetrafluoroethylene aqueous dispersion composition according toclaim 1 wherein the nonionic surfactant has a molecular structurerepresented by the Formula (1) wherein x is from 10 to 16, and A is apolyoxyalkylene chain comprising from 7 to 12 oxyethylene groups andfrom 0 to 2 oxypropylene groups.
 3. The polytetrafluoroethylene aqueousdispersion composition according to claim 1 wherein the nonionicsurfactant has a molecular structure represented by the Formula (2)wherein x′ is from 4 to 12, and A′ is a polyoxyalkylene chain comprisingfrom 7 to 12 oxyethylene groups.
 4. The polytetrafluoroethylene aqueousdispersion composition as claimed in claim 1, wherein the nonionicsurfactant is C₁₀H₂₁CH(CH₃)CH₂O(C₂H₄O)₉C₃H₆OH.
 5. Thepolytetrafluoroethylene aqueous dispersion composition as claimed inclaim 1, wherein the nonionic surfactant is C₁₃H₂₇O(C₂H₄O)₈C₃H₆OH orC₁₃H₂₇O(C₂H₄O)₁₀(C₃H₆O)₂H.
 6. The polytetrafluoroethylene aqueousdispersion composition as claimed in claim 1, wherein said thickener isobtained by reacting a polyisocyanate with a polyalkylene ether alcoholor by reacting a polyisocyanate with a polyalkylene ether polyol and apolyalkylene ether alcohol.
 7. The polytetrafluoroethylene aqueousdispersion composition as claimed in claim 1, wherein the thickener hasthe following formula:

wherein R¹ is a m-valent hydrocarbon group, each of R² and R⁴ may be thesame or different and are alkyene groups, R³ is a (h+1) valenthydrocarbon group containing a urethane bond, R⁵ is a branchedhydrocarbon group, m is at least 2, h is at least 1, each of k and n isfrom 1 to 500, and R² and R⁴ are ethylene groups.
 8. Thepolytetrafluoroethylene aqueous dispersion composition as claimed inclaim 1, wherein the thickener is represented by the following formula:

wherein R⁶ is a q-valent hydrocarbon group which may contain a urethanebond, R⁷ is an alkylene group, R⁸ is a branched hydrocarbon group, p isfrom 1 to 500, q is at least 1, and R⁷ is an ethylene group.
 9. Thepolytetrafluoroethylene aqueous dispersion composition as claimed inclaim 1, wherein an average particle size of the fine particles ofpolytetrafluoroethylene is from 0.15 to 0.3 microns.
 10. Apolytetrafluoroethylene aqueous dispersion composition consistingessentially of from 30 to 65 wt % of fine particles ofpolytetrafluoroethylene obtained by emulsion polymerization, and from 2to 12 wt %, based on the polytetrafluoroethylene, of a nonionicsurfactant having a molecular structure represented by the Formula (3):R—O—B—H  (3) wherein R is an alkyl group having a straight chain or abranched chain represented by C_(x)H_(2x+1) where x=8 to 18, and B is apolyoxyalkylene chain comprising from 5 to 20 oxyethylene groups andfrom 1 to 2 oxypropylene groups, wherein the one or two oxypropylenegroups are bonded to a terminus of the polyoxyalkylene chain.
 11. Thepolytetrafluoroethylene aqueous dispersion composition according toclaim 10, wherein the nonionic surfactant has a molecular structurerepresented by the Formula (3) wherein x is from 10 to 16, and B is apolyoxyalkylene chain comprising from 7 to 12 oxyethylene groups andfrom 1 to 2 oxypropylene groups.